Apparatus for generating sprays of droplets of controlled size in useful investigations of the combustion of liquid fuels and for other purposes. An impulse generator suitable for such uses was described in the inventors' paper, "DEVELOPMENT OF MULTI-ORIFICE IMPULSED SPRAY GENERATORS FOR HETEROGENEOUS COMBUSTION EXPERIMENTS", published on Mar. 20, 1983 in ASME/JSME Thermal Engineering Joint Conference Proceedings--Volume Two (pp. 433-439). That generator illustrated in FIG. 1 of the paper comprised a relatively flat reservoir to the bottom surface of which was affixed a piezoelectric crystal. The upper surface or plate was pierced with orifices. When the reservoir was filled with fluid and the crystal activated with electric pulses of controlled frequencies, streams of droplets were ejected from the orifices. The droplet size depended on the diameter of the orifices and the frequency of the electric pulses. Although that generator proved to be satisfactory in most respects, we observed that after continued use, some of the orifices became obstructed by a liquid film due to the leakage from imperfect orifices on the plate. Other disadvantages of the previous system are the difficulty of controlling the trajectories of the droplets if the orifices are not pierced perfectly on the plate, and the difficulty of repairing imperfect orifices. Also, the previous system is not well adapted to distributing gas into the spray.